Conventionally, a two-stroke engine is provided with a scavenging port that communicates a side portion of the interior of the cylinder with the crank chamber, so that an air-fuel mixture containing fuel is supplied from the crank chamber into the cylinder via the scavenging port, and this flow of air-fuel mixture displaces or scavenges the combustion gas remaining in the cylinder out of the combustion chamber at the same time. The scavenging orifice at the downstream end of the scavenging port is opened and closed depending on the position of the piston that reciprocates in the cylinder such that the scavenging orifice communicates with the combustion chamber defined in an upper part of the cylinder when the piston is near the bottom dead center, and is shut off by the piston skirt when the piston is near the top dead center.
In such a two-stroke engine, it is known to perform stratified scavenging by providing a scavenging passage in addition to the air-fuel mixture passage (see JP3143375B, for example). In JP3143375B, as shown in FIG. 9A for example, the two-stroke engine is provided with an air-fuel mixture passage 160 that supplies air-fuel mixture to a crank chamber 102A, an air supply passage 157 that supplies air to a scavenging passage 156 extending from a crank chamber 102A to a cylinder 122, an air flow passage 161 located on an upstream side of the air-fuel mixture passage 160 and the air supply passage 157 and connected to both of the air-fuel mixture passage 160 and the air supply passage 157, and a check valve 154 provided in the air flow passage 161, whereby stratified scavenging is performed.
Namely, in this structure, as shown in FIG. 9B, when a piston 123 moves upward, the pressure in the crank chamber 102A decreases and the air-fuel mixture enters the crank chamber 102A via the air-fuel mixture passage 160 while the air enters the crank chamber 102A via the air supply passage 157 and the scavenging passage 156. As shown in FIG. 9C, when the piston moves downward, the pressure in the crank chamber 102A increases, and the air held in the scavenging passage 156 enters the cylinder 122 first, and then, the air-fuel mixture held in the crank chamber 102A is supplied to the cylinder 122, to scavenge out the combustion gas remaining in the cylinder 122. In this way, stratified scavenging is performed, and this prevents an uncombusted air-fuel mixture from flowing out to an exhaust port 131 during the scavenging and thereby suppresses an increase in total hydrocarbons (THC).
However, in such stratified scavenging, it is assumed that the scavenging orifice 155 is closed by the piston side surface when the piston 123 is positioned near the top dead center. Therefore, in a case where a structure with a long piston stroke is adopted to improve the thermal efficiency and as a result the scavenging orifice 155 communicates with the crank chamber 102A via a part of the cylinder 122 below the lower end of the piston skirt when the piston 123 is positioned near the top dead center as shown in FIG. 10A, the stratified scavenging cannot be performed.
In other words, in such a structure, as shown in FIG. 10B, when the pressure in the crank chamber 102A decreases during an upward movement of the piston 123, the air-fuel mixture enters the crank chamber 102A via the air-fuel mixture passage 160 while the air enters the crank chamber 102A directly from the air supply passage 157 through the scavenging orifice 155 without flowing to the scavenging passage 156. Consequently, as shown in FIG. 10C, when the pressure in the crank chamber 102A increases during a download movement of the piston 123, first the air-fuel mixture held in the scavenging passage 156 and then the air-fuel mixture in the crank chamber 102A enter the cylinder 122. Therefore, the uncombusted air-fuel mixture may be discharged through the exhaust port 131 during the scavenging of the combustion gas in the cylinder 122.
It may be conceived to extend the length of the piston skirt to close the scavenging orifice 155 with the piston skirt when the piston 123 is positioned near the top dead center. However, if such a structure were adopted, the piston skirt would come to contact with other component parts (such as a counterweight of the crankshaft) easily when the piston 123 is positioned near the bottom dead center, and in addition, the weight of the piston 123 would increase.
In view of the aforementioned background, an object of the present invention is to make it possible to perform stratified scavenging in a two-stroke engine even when a long piston stroke is adopted.